A grow tent is a tent made exactly for the purpose of growing plants. Often times these are tailored precicely to the purpose of growing marijuana indoors. They have the appropriate measurements and room to spare to fit air filters, lighting and such.
One of the biggest benefits of a grow tent is that you have full control over the environment in the tent. This makes controlling the lightcycle, humidity, smell and heat a lot easier.
More grow room definitions:
Grow room | Grow box | Security | Anti detection foil | Sound proof | Pond liner | Green house